Regulating system for thermionic devices



W. K. KEARSLEY REGULATING SYSTEM FOR THERMIONIC DEVICES Filed March 24,1927 Fig.1.

Inventor: William K. Kearsle g,

His Attorney.

Patented Sept. 4, 1928.

UNITED STATES PATENT OFFICE.

WILLIAM K. KEABSLEY, SCBIENECTADY, NEW YORK, ASSIGNOR TQ GENERAL ELEC-TRIC COMPANY A. CORPORATION OF NEW YORK.

REGULATING SYSTEM FOR THERMIONIC DEVICES.

Application and March 24, 1927. Serial No. 178,106.

The present invention relates to thermionic apparatus, and in particularto the regulation of thermionic X-ray tubes and its object is to providea stabilizer or regulater for maintaining a desired electron emissivityand thereby a desired space current in a thermionic device.

The X-ray output of an X-ray tube varies directly with the electronemission of the cathode.w The electron emission varies both withvariations of cathode temperature and variations of gas conditions inthe X-ray tube. When an X-ray tube is operated by current derived fromcommercial circuits,

variations in voltage are constantly occur ring thereby altering thecathode tempera ture. Also ordinary X-ray tubes are subject to changesin residual gas content.

Tubes containing targets backed with copper or other metal, which can befreed from gas only with great difiiculty, are particularly subject tosuch changes.

It has been found desirable, therefore, to provide a stabilizer wherebythe electron emission of such a thermionic device as a Coolidge X-raytube is maintained constant even though its gas content or the supplyvoltage for the cathode of the tube should vary. In my prior UnitedStates patent No. 1,653,102, issued December 20, 1927, and in a paperpublished by me in the Journal of Radiology for July, 1921, I havedescribed a vibrating stabilizer whereby in response to. the spacecurrent in an X-ray tube the duration of the flow of heating current forthe cathode is so controlled by mechanical make and break contactsduring the half wave im ulses when current flow occurs through t e X-raytube that the electron emission is maintained at the desired value.

In accordance with my present invention the control of the cathodeheating current in the X-ray tube, or other thermionic device to beregulated, is carried on by an auxiliary thermionic device which isprovided with means, such as a grid, for controlling the duration offlow of space current therein. This auxiliary tube operates inconjunction with regulating means to so vary the voltage of the heatingcurrent for the cathode of the main device that a desired space currentis maintained in the main device. The novel" feature of my inventionwill be pointed out with greater particularity in the appended claims.

The accompanying drawing shows in Fig. 1 a diagrammatic illustration ofmy invention as applied to an X-ray tube, and Fig. 2 illustrates amodification. a

In the system shown in Fig. 1, the thermionic device in which apredetermined space current is desired is indicated at 1. This device,for illustration, maybe ahi hly exhausted X-ray tube, such as theCoolldge tube, which is described in U. S. Patent 1,203,495 issuedOctober 31, 1916. This tube is provided with a thermionic cathode 2 andan anode 3, theseelectrodes being connected in the usual manner tocurrent supply conductorsd, 5. The cathode 2 is supplied with heatingcurrent by the conductors 4, 6 which are connected to the secondar of atransformer 7. The prima winding of this transformer is connecte by theconductors 8, 9 to the supply mains 10, the cur rent in the main circuit4, 5 and in the heating circuit 4, 6, therefore, being in synchronism.The mainsupply conductors 4, 5 for the X-ray tube are connected to thehigh potential secondary of a transformer 12. The primary winding of thetransformer 12 is connected by the conductors 13, 14 in circuit with aswitch 15 to the supply mains 10. The secondary windings 16, 17 of thetransformer 12 are connected in series through a non-inductiveresistance'18, which is shunted by a condenser 19. The condenser 19prevents transient current impulses from starting the operation of thedevice 21 prematurely.

Assuming the device 1 to be operated at saturation current for any givencathode temperature, which is the usual method of operating a thermionicX-ray tube, then in the system described the space current through thetube would vary with the variation of voltage in the supply mains 10 orwith a variation of gas content of the tube. In order to prevent suchvariation of space current, I have provided a thermionic regulator tube21. As diagrammatically indicated, the tube 21 has a thermionic cathode22, an anode 23 and a' grid 24,-, which has been re resented by a dottedline and which preferably surrounds-the anode. This auxiliary tubeshould be first highlly evacuated and freed of water vapor an y thenprovided with a as or a source of vapor, such as mercury. t may beprovided with some inert gas, such as argon, at a pressure, for example,of about five and one-half centimeters of mercury. An electrical valvetube of this general character is known as a th ratron.

n a thyratron the current flow is initiated suddenly from zero tofullnormal value when the char e of the grid changes from negative topositive. Should the grid charge become ne ative while space current isflowing throng a thyratron, no change of space current results.

' The cathode 22 of this valve tube is supplied with heatin currentby atransformer 26, the primary 0 which is connected by the conductors 27,28 to the sup 1 mains 10. The cathode and the anode of the regulatortube 21 are. connected by the conductors 29, 30 to the primary circuitof the sup 1y transformer 7 and receive current from t e supply mains 10through a non-inductive resistance 31. The grid 24 and the cathode 22are connected respectively by the conductors 32, 33 to the resistance18, one of the terminals 34 referably being adjustable, as indicated. Aiasing battery 35 is provided in the grid circuit 3 33 and is connectedas indicated to impress a negative potential on' the grid 24.

In the system described a space current through the thermionic tube 1produces a drop of voltage in the resistance 18 which is connected to opose in potential the biasing battery 35. As ong as the grid potential isnegative no current flow will take place throughthe valvetube 21. Whenthe space current through the tube 1 increases in any half wave impulseto a value at which the potential drop across the resistance 18 exceedsthe potential drop across the battery 35, then t e grid becomes positiveand an electronic current flow begins through the valve tube 21. Asthecurrent which flows through the valve tube 21 is derived through theresistance 31, an increase of current through this valve tube increasesthe potential drop in the resistance 31 and lowers.the

voltage of the heating current for the cath-' ode 2 of the mainthermionic tube 1. The tem rature of the cathode thereby is loweredecreasing the electron emission or preventing its undesired increaseand conseuently maintaining the space. current t rough the thermionictube 1 at a desired average value. 7

An tendency for the. currentin thetube 1 to all below a desired value,for example due to a decrease of voltage in. the supply mains 10, 11, orto the evolution of deleterious gas, is immediatel counteracted throughthe intermediary of a regulator tube 21 l y a decrease of voltage dropin the resistance 31 and an increase of cathode temperature so that theright amount of heating current is delivered to the cathode 2 tomaintain the space current at the desired value.

The 0 eration of the regulator tube 21 occurs witiiout time la and thesystem is free from all mechanica ly moving parts and therefore remainsat all times in proper working condition without mechanical adjustments.The regulation is so quickly responsive to conditions and is so positivein its action that no perceptible variation. of average space currentoccurs in the circuit of the main tube 1 as shown by a meter. Duringeach half wave of alternating current passing through the mainthermionic tube the voltage of the heating current is so regulated thatthe electron emission of the cathode in the main device is maintained atthe desired value.

The system shown in Fig. 2 is similar in most respects to the s stemshown in Fig. 1

but differs therefrom y the insertion of the secondary winding of thetransformer 37 in place of the biasing battery 35. The primary windingof this transformer 37 is connected by the conductors 38, 39 to theconductors 13, 14 which sup 1y current to the main high potentialtransformer 12. It also differs from the system shown in Fig. liby theuse of an inductance coil 40 as an impedance device in lace of thenon-inductive resistance 18. he potential of the inductance coil 40 isop osed to the potential of the secondary of t a transformer 37.- Upon arise of current in the main circuit 4, 5, the potential of the grid 24of theregulator tube21 becomes ositive so that current flow beginsthrough t e regulatortube. Current through the regulatortube will occurduring periods of such duration that the space current through the tube1 is maintained at the desired predetermined value. Should the s acecurrent in themain tube tend to rise,

ow of current throughthe regulator tube will occur earlier during thecycle and should the space current therein tend to fall then currentflow through the regulator tube will occur later to maintain the voltageof the heatin current at .a value which will produce a esired electronemission from the cathode of the tube 1. t

In the operation of either of the systems shown in igs."1 and 2 noperceptible departure from a predetermined value of the lli lzo

of the space current in an X-ray tube in order to regulate its X-rayoutput, I wish it to be understood that it is of general application tothermionic devices in which a regulation of space current is desired.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The combination of a main thermionic device, means for deliveringimpulses of current to the cathode of said device, thermionic regulatingmeans for decreasing said current impulses, means for initiating theoperation of said regulating means when the space current in said mainthermionic device has reached a predetermined value thereby opposingfurther increase of space current.

2. The combination of a thermionic X-ray tube, means for deliveringvimpulses of current to the cathode of said tube, thermionic regulatingmeans for controlling said current impulses, means for rendering saiddevice non-operative during a part of each such impulse of current, andmeans put into operation when the space current in said X-ray tubereaches a predetermined value for rendering operative said thermionicmeans.

3. The combination of a thermionic tube to be regulated, a heatingcircuit for the cathode of said tube, a voltage-consuming means in saidheating circuit, an auxiliary thermionic device which is provided with agrid and contains a supply of gas at a pressure sufliciently high toneutralize space charge by ionization, electrical connections wherebyspace current will be delivered to said auxiliary device in circuit withsaid volta e-consuming means, and means whereby sald grid will becharged to a positive potential when the s ace current through the tubeto be regulate rises to and above a predetermined value.

4. The combination of a thermionic X-ray tube, a circuit for deliveringcurrent to heat the cathode of said tube, a voltage-consuming means insaid circuit, an auxiliary thermionic device of the gas ionization typeprovided with a grid for preventing the initiation of current flowtherethrough in response to a negative potential applied to said grid,circuit connections for deriving current for said auxiliary devicethrough said voltageconsuming means, means for biasing said grid at anegative potential and means for opposing said negative potential by apositive potential varying directly in response tobvariations in spacecurrent in said X-ray tu e.

5. The combination of a main thermionic device, a heating circuit forthe cathode of said device, voltage-consuming means in said circuit, anauxiliary thermionic device connected to vary the voltage drop in saidvoltage-consuming means, and means for timing the flow of space currentin said auxiliary thermionic device to occur when the space current ofthe main thermionic device has reached a redetermined value.

6. An -ray apparatus comprising an X- ray tube having a cathode adaptedto be heated by passage of current, a source of alternating currentvconnected to the main electrodes of said X-ray tube, a heating circuitfor the cathode arranged to receive alternating current in synchronismwith said source, and thermionic regulating means responsive to loadcurrent in said X-ray tube, means for rendering said means non-operativeat the beginning of successive current impulses through said X-ra tube,and means for withdrawing current y said thermionic means from saidheating circuit for periods of such duration that the electron emissionof said cathode is maintained at a substantially constant average value.

7. An electric apparatus comprising a thermionic device, a cathodeheating circuit therefor, a thyratron, circuit connections fordelivering impulses of variable current to said device, means forinitially charging the grid of said thyratron to a negative potential,means for superimposing a positive potential upon said grid varyingdirectly with the current through said thermionic device and circuitconnections for regulating the electrical heating current of the cathodeof said thermionic device inversely with current flow through saidthyratron.

8. An X-ray apparatus comprising a thermionic pure electron dischargeX-ray tube, a source of alternating current connected to the mainelectrodes of said tube, an impedance device in said circuit, a cathodeheating circuit also connected to said source, a thyratron having acathode, an anode and a grid, connections from the grid and cathode ofsaid thyratron to said impedance device whereby an increase of currentin said X-ray device produces an increase of length of the conductiveperiods of said thyratron and circuit connections from the cathode andanode of said thyratron whereby current is withdrawn therethrough fromthe heating circuit of said X-ray tube.

In witness whereof, I have hereunto set my hand this 23d day of March,1927.

WILLIAM K. KEARSLEY.

